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Bureau of Mines Information Circular/1987 



Water-Jet-Assisted Drag Bit Cutting 
in Medium-Strength Rock 

By J. E. Geier, M. Hood, and E. D. Thimons 



UNITED STATES DEPARTMENT OF THE INTERIOR 



Information Circular 9164 



Water-Jet-Assisted Drag Bit Cutting 
in Medium-Strength Rock 

By J. E. Geier, M. Hood, and E. D. Thimons 



UNITED STATES DEPARTMENT OF THE INTERIOR 
Donald Paul Hodel, Secretary 

BUREAU OF MINES 

David S. Brown, Acting Director 







yf»' 



Library of Congress Cataloging in Publication Data: 



Geier, J. E. (Joe E.) 

Water-jet-assisted drag bit cutting in medium-strength rock. 

(Information circular; 9164) 

Bibliography: p. 9-10 

Supt. of Docs, no.: I 28.27: 9164. 

1. Drag bits (Drilling and boring) 2. Jet cutting. 3. Water-jet. I. Hood, M. (Mike) II. 
Thimons, Edward D. III. Title. IV. Series: Information circular (United States. Bureau of 
Mines); 9164. 



TN295.U4 [TN281] 



623 s 



[622'.23] 



87-600274 



CONTENTS 

Page 

Abstract 1 

Introduction 2 

Theories of water-jet assistance 2 

Lubrication 3 

Stress corrosion cracking 4 

Suppression of secondary chipping 4 

Hydraulic wedging 6 

Erosion of crushed rock 7 

Conclusions 9 

References 9 

ILLUSTRATIONS 

1. Typical chip formation cycle 4 

2. Results of size analyses of chips formed by cutting in Indiana limestone 

with and without water-jet assistance 5 

3. Samples of broken rock 5 

4. Plots of F c versus distance traveled by bit for typical chip formation 

cycles 7 

5. Force reductions achieved in Indiana limestone for 15-mm-deep cuts 8 

6. Force reductions achieved in Grindleford sandstone for 10-mm-deep cuts 8 





UNIT OF MEASURE ABBREVIATIONS 


USED IN 


THIS REPORT 


cm 2 


square centimeter 


kJ/m 


kilojoule per meter 


cm/s 


centimeter per second 


kN 


kilonewton 


deg 


degree 


m 


meter 


J 


joule 


mm 


millimeter 


J/m 


joule per meter 


MPa 


megapascal 


J/m 2 


joule per square meter 


m/s 


meter per second 


J /ram 


joule per millimeter 







WATER- JET-ASSISTED DRAG BIT CUTTING IN 
MEDIUM-STRENGTH ROCK 

By J. E. Geier, 1 M. Hood, 2 and E. D. Thimons 3 



ABSTRACT 

This Bureau of Mines report reviews hypotheses for the mechanism by 
which water jets reduce the specific energy of cutting for drag bits. 
Several of these hypotheses are shown to be inconsistent with published 
evidence and new observations. The notion of a limiting cutting speed, 
above which water jets would be incapable of rendering assistance, is 
shown to be improbable. The hypothesis that seems most plausible is 
that specific energy reductions are mainly due to the erosion of crushed 
materials from in front of the bit. This hypothesis leads to a predic- 
tion that, for a given rock type and jet-bit configuration at a constant 
depth of cut, the reduction in bit forces should be a function of dW/dx, 
the jet energy spent per unit length of cut. This dependence upon dW/dx 
is shown to be consistent with other workers' results. 

Graduate student, University of California, Berkeley, CA. 
^Associate professor, University of California, Berkeley, CA. 

^Supervisory physical scientist, Pittsburgh Research Center, Bureau of Mines, 
Pittsburgh, PA. 



INTRODUCTION 



The use of water jets to augment mec- 
hanical rock cutting has advanced greatly 
since the discovery a decade ago (1_)„ 
that water jets diminish the forces act- 
ing upon drag bits cutting in strong 
rock. Other research workers cutting in 
a wide variety of rock types and with 
various geometries of bits unanimously 
confirmed that substantial benefits to 
the cutting process, including signifi- 
cant force reductions, are observed under 
a broad range of cutting conditions (2- 
4_) . Following these early, mainly lab- 
oratory, studies, prototypical systems 
using this technology were tested, and 
many of the benefits predicted from the 
laboratory investigations were realized 
in the field. In particular, the use of 
water jets has been demonstrated to en- 
hance ability to cut harder rocks, reduce 
bit wear, diminish machine vibrations (_5, 
6_) , and dramatically reduce the dust pro- 
duced at the face (_Z_~~_9) . However, ob- 
servations on the effects of water jets 
on cutting efficiency have been seemingly 
contradictory. For example, Fairhurst 
(10) and Kovscek (_7) claim that, at the 
high bit velocities (1 to 3 m/s ) typical 
of rotating-drum machines such as shear- 
ers, the benefit of reduced bit forces 
decreases, in some cases to zero. These 
findings appear to conflict with results 
from the field trials with roadheaders, 
where the forces acting were decreased 
substantially. 

Commercial development of this tech- 
nology is greatly hindered by this incom- 
prehension of the water-jet assistance 
mechanism, both because design parameters 
such as jet position and pressure cannot 
be optimized via a quantitative theory 
and because there is no way of knowing 



which parameters to consider in empirical 
optimization studies. Hence if the full 
benefits of water-jet assistance are to 
be realized, a better physical under- 
standing of the assistance mechanism is 
essential. 

Since the mechanism of water-jet assis- 
tance is still poorly understood, the 
only guide for the commercial development 
of this technology is a body of labora- 
tory data that is deficient in at least 
three respects: 

1. The data have been gathered from 
cutting tests in an assortment of rock 
types, using bits of various geometries 
and employing a variety of jet-bit con- 
figurations. These data are not neces- 
sarily useful for predicting water-jet 
effectiveness in other rocks, using bits 
and jet-bit configurations other than the 
ones specifically tested. 

2. Although laboratory studies have 
measured the effects of individual pa- 
rameters such as jet pressure and cutting 
speed, in general these studies have 
considered only one or two of these 
parameters. 

3. For many of these studies, the 
statistical significance of the results 
has not been assessed. These deficien- 
cies arise largely because, in the ab- 
sence of an established mechanistic 
theory, so many parameters must be con- 
sidered that a comprehensive, empirical 
study is not practical. An improved 
understanding of the mechanism of water- 
jet assistance would reduce the number of 
parameters to be considered and thus 
facilitate compilation of a more complete 
and reliable set of data which could be 
used for design purposes. 



THEORIES OF WATER- JET ASSISTANCE 



In the present paper, the term "water- 
jet assistance" refers to the degree to 
which the mechanical portion of cutting 

Underlined numbers in parentheses re- 
fer to items in the list of references at 
the end of this report. 



energy is reduced when high-pressure (10- 
to 70-MPa) water jets are used in con- 
junction with drag bits. Since the mec- 
hanical portion of the cutting specific 
energy E s is 

Es = df I F * V dt > 



where F is the force applied to the rock 
by the bit, v is the bit velocity or 
cutting speed, t is the time, and V is 
the volume of rock excavated by the bit, 
this reduction in specific energy may be 
manifested as reduced components of F, if 
the volume excavated per unit length of 
cut, dV/dx, is held constant, or as in- 
creased cutting rates, if F is held con- 
stant. In general, F is more directly 
measurable in the laboratory than is E s , 
and so it is convenient to speak of jet 
assistance in terms of the force re- 
ductions observed when dV/dx is held 
approximately constant. 

Most researchers on the subject have 
measured water-jet effectiveness in terms 
of the difference between bit forces 
measured for identical bits cutting with 
and without the aid of water jets. Mor- 
ris (11 ) suggested that a more realistic 
measure of water-jet effectiveness would 
be the rate at which forces increase due 
to bit wear over the life of the bit. 
This is true, if the objective of a study 
is simply to optimize a particular water- 
jet-assisted cutting system, or to demon- 
strate the benefits that can be achieved 
through the use of water jets. However, 
if the objective is to determine the 
precise mechanism of water-jet assis- 
tance, this approach is inappropriate, 
since it would confound the long-term ef- 
fects of bit wear with the instantaneous 
effect of a water jet in reducing E s . 

While the observation that water jets 
reduce bit wear may explain the greater 
portion of the force reductions seen over 
the life of a bit, it does not explain 
why force reductions are observed when 
water jets are used to assist bits in any 
state of wear. Furthermore, as argued by 
Cook (12) , wear occurs largely due to the 
heat generated on the bit wearflat by the 
frictional force, which is proportional 
to the normal force. Thus if the water 
jets reduce bit forces for a given depth 
of cut, wear reductions may occur due to 
the force reductions as well as the cool- 
ing mechanism. For these reasons, the 
present paper examines water-jet assis- 
tance in terms of the instantaneous ef- 
fect of a water jet in reducing E s . In 
the following sections, five hypotheses 



on the phenomenon of water-jet assis- 
tance are reviewed. 

LUBRICATION 

A notion that persists in the litera- 
ture is that the force acting upon a bit 
is reduced by water jets because the 
water behaves as a lubricant, allowing 
the bit to slide more easily over the 
rock surface. This is equivalent to sup- 
posing that the coefficient of friction 
between bit and rock is reduced by the 
water jets. 

This hypothesis is clearly inconsistent 
with the work of Hood ( 13 ) , who measured 
the forces acting on a drag bit of nega- 
tive rake angle, cutting with and without 
water jets. High-speed photography 
showed that only the bottom edge (i.e., 
the wearflat) of the bit was consistently 
in contact with the rock. Hence the co- 
efficient of friction y e between rock and 
bit was the ratio of the cutting force F c 
(the component of F in the direction of 
bit travel) to the normal force F n (the 
component of F normal to the rock sur- 
face). It was found that the use of 
water jets reduced F n by a greater 
percentage than F c , and thus the water 
jets effectively increased y e in these 
experiments. 

For a bit of positive rake angle, it is 
more difficult to isolate the effect of 
water jets on y e » since for such a bit 
the front face is generally in contact 
with the rock, as in figure 1, and hence 
F c includes both a frictional component 
and a "plowing" component. However, 
experiments with such bits (2^ J^4) have 
consistently shown that F n is reduced by 
a greater percentage than is F c . Thus 
there is no reason to suspect that the 
water jets reduce the coefficient of 
friction seen by these bits. 

An explanation of the observed increase 
in y e is that the water jet washes away 
the crushed rock that forms ahead of the 
bit, and therefore the bit slides on the 
rock surface rather than on a thin layer 
of crushed rock between the rock and the 
bit. This layer would presumably behave 
as a lubricating soil beneath the bit, 
increasing F n but decreasing \i e > by 



VAWAV/A 

Intact rock 




B 



WXWA 



Crushing ahead 
of bit 




Increased 
crushing 



'^$$£$ffiizi0i3> 





vrnmj^ 



FIGURE 1.— Typical chip formation cycle. A, crushed debris ahead of bit after formation of a major chip; S, crushing of intact 
rock just ahead of bit as the bit advances; C, formation of minor chips and initiation of fractures; D, propagation of one of these 
fractures to form the next major chip. 



washing away 
would remove 
crease the 
friction. 



this layer, the water jet 
the lubricant and thus in- 
effective coefficient of 



STRESS CORROSION CRACKING 

The possibility that water introduced 
by the water jets might lower the frac- 
ture strength of the rock, by chemical at- 
tack was investigated by Tutluoglu (15). 
He determined that the maximum crack 
propagation speed at which this mechanism 
could influence the energy needed to 
drive the cracks was about two orders of 
magnitude less than the crack propagation 
speeds that occur during drag bit cut- 
ting. Hence this mechanism, known as 
stress corrosion cracking, could not play 
a significant role in water-jet-assisted 
cutting. 

SUPPRESSION OF SECONDARY CHIPPING 

The process by which a drag bit cuts 
through rock is known to be cyclic in 
nature. When water jets are not used, 
the typical chip formation cycle occurs 
as shown in figure 1. After a large chip 
forms, a certain amount of crushed rock 
remains in the path of the bit. As the 
bit advances into the intact rock beneath 



this debris, more debris is created by 
crushing and by the formation of smaller 
chips, until eventually a second large 
chip forms. 

Recently the suggestion was made (16) 
that water jets might improve the ef- 
ficiency of the cutting process by sup- 
pressing formation of the smaller rock 
chips prior to formation of the large 
primary chips. However, laboratory evi- 
dence indicates that water jets do not 
significantly suppress this secondary 
chip formation. Tutluoglu ( 15 ) performed 
size analyses on the broken rock col- 
lected from cuts made with and without 
water jet assistance, using chisel-type 
bits in Indiana limestone. The results 
of these size analyses (fig. 2) indicated 
that there is no significant difference 
between the size distributions of broken 
rock formed by the two processes. This 
finding is consistent with the authors' 
qualitative observation that, over a 
range of jet pressures, flow rates, and 
cutting speeds, there is no obvious vari- 
ation in the size distribution of the 
chips that are formed. Figure 3 shows 
typical samples of the broken rock that 
were collected after making 15-mm-deep 
cuts in Indiana limestone, using the same 
type of bit as was used by Tutluoglu 
(15). 



S2 | 



LU w- 
> 2 

< «_ 
_J o 

O 



00.0 



10.0 - 



.0 



0, 



KEY 

• With jets 
o Without jets 




0.001 



0.010 



0.100 



.000 



10.00 100.0 



SIZE, mm 



FIGURE 2.— Results of size analyses of chips formed by cutting in Indiana limestone with and without water-jet assistance. 






FIGURE 3.— Samples of broken rock from cuts made A with no water-jet assistance, B with dW/dx = 12 J/mm, and C with 
dW/dx = 47 J/mm. 



The force exerted by the water jet on 
the rock surface has also been proposed 
as a mechanism whereby water jets dimin- 
ish the cutting efficiency when the cut- 
ting speed v is sufficiently high that 
the water jet cannot penetrate the rock. 
That a pressure on the upper surface of 
rock increases the energy needed to form 
chips is a well-known phenomenon in deep 
drilling, where it is referred to as chip 



holddown. When a water jet strikes the 
upper surface of a forming rock chip, but 
does not penetrate the chip, the water 
jet exerts on the chip a downward force 
F, 



WJ 



F W j ~ 



a 2 p. 



where p is the jet pressure and a 
jet diameter. 



is the 



Fairhurst (10) suggested that chip 
holddown due to F w j may be important in 
water-jet-assisted cutting, at v suffi- 
ciently high that the water jet cannot 
penetrate the rock. This mechanism is 
invoked to explain the observations that, 
at v > 1.4 m/s or so, water jets appeared 
to produce an increase in bit forces. 
However, this mechanism requires that, 
with increasing v, the peak bit forces 
should increase by a greater percentage 
than the mean forces, since this mechan- 
ism would act during the formation of the 
large chips, if at all. This is not seen 
in the published data. Moreover, in 
those experiments little force reduction 
was observed at any cutting speed, and it 
is difficult to say whether the trends 
cited are significant relative to the 
unexplained variance apparent in the 
published graphs of the average cutting 
and normal forces. 

The present authors speculate that the 
force reductions seen by Fairhurst (10) 
may have been low due to the 20° angle 
between the water jet and the bit face, 
which may have caused much of the jet 
energy to miss or be deflected away from 
the narrow crushed zone ahead of the bit, 
and to the low levels of dW/dx (4 to 16 
J/mm). The critical importance of water- 
jet position with respect to the bit has 
been established by Tutluoglu (14). 

HYDRAULIC WEDGING 

A fourth hypothesis to explain the re- 
duction in mechanical specific energy E s 
when water jets are used is that water 
pressure aids in driving the cracks that 
form the major rock chips. This mecha- 
nism was proposed by Hood ( 13) , who per- 
formed experiments using a sliding 
indenter as the cutting tool. A tensile 
crack chip can be initiated at relatively 
low indenter loads, but propagation of 
this crack takes place only after much 
mechanical energy is spent in crushing 
the rock beneath the indenter (17). Hood 
(13) proposed that the reduction in E s 
that is seen in cutting experiments with 
water jets occurs because the water jets 
penetrate the cracks that form at low 



levels of bit force, and then drive these 
cracks without the need for further 
crushing and consequent wasting of energy 
beneath the bit. Quasi-static indenta- 
tion tests confirmed that chips do form 
at substantially reduced indenter force 
levels when water jets are employed. 

This hydraulic wedging mechanism of 
water-jet assistance, while it may be 
important for indentation tools such as 
blunt drag bits and disc cutters, is 
probably not significant for sharp drag 
bits. (A sharp drag bit is considered to 
be one for which F c >>F n , and a blunt drag 
bit is one for which F c <F n .) With a 
blunt bit, crushing takes place predomi- 
nantly beneath the wearflat; indeed, as 
noted earlier, the leading face of a 
blunt bit is often not in contact with 
the rock, in which case crushing can oc- 
cur only beneath the wearflat. With 
sharp bits, however, crushing takes place 
mainly ahead of the leading face of the 
bit. Observations of the cutting process 
(fig. 1) show that after a large chip is 
created, the area of rock confronting the 
bit is not sufficient to transmit the 
high force needed to drive a crack and 
produce a second large chip. Hence as 
the bit moves forward, it crushes the 
rock until the area confronting the bit 
is sufficient for a second large chip to 
be produced. Much energy must be spent 
on compressing, shearing, and recrushing 
the debris thus formed as the bit ad- 
vances, especially if the debris is con- 
fined in front of the bit. 

Examination of force versus distance 
plots for typical chip formation cycles 
with and without water-jet assistance 
(fig. 4) shows that the dominant influ- 
ence of the water jets is to reduce the 
energy spent on this crushing stage. As 
evidenced by the shaded areas (which 
represent the mechanical energy), the 
energy needed to drive a crack after it 
begins to propagate is small relative to 
the energy spent on crushing. Indeed, a 
simple calculation shows that the energy 
needed to drive the crack to form a large 
chip is trivial; therefore, if this were 
the major influence of the water jet, the 
reduction in E s would be minuscule. For 



KEY 

Energy required to form chip 
Energy spent in crushing 



role of the hydraulic wedging mechanism 
must be minor. 

EROSION OF CRUSHED ROCK 



Ld 
O 

o 



h- 
f- 

3 
O 




12 
10 

8 
6 



Without jets 

Formation of 
major chip 




Wmm 

aJHI 



5 6 

DISTANCE, cm 



7 



FIGURE 4.— Plots of Fc versus distance traveled by bit for 
typical chip formation cycles observed with jets (dW/dx = 19 
J/mm) and without water-jet assistance. 



example, wet Indiana (Salem) limestone 
has a fracture energy R = 40 J/m 2 (18). A 
15-mm-deep cut in this rock produces 
chips that generally have a surface area 
measuring less than 50 cm , so the for- 
mation of a large chip requires an energy 
expenditure of about 0.2 J. Over a dis- 
tance of 1 m, about 25 chips of this 
order of size are formed, so the energy 
spent in producing these chips is about 5 
J/m. As reported by Tutluoglu (15), the 
total energy spent in making such a cut 
is about 4.5 kJ/m. Thus it may be con- 
cluded that, for sharp drag bits, the 



The most promising explanation of 
water-jet assistance is the hypothesis 
that the water jet removes crushed rock 
that would otherwise interfere with the 
advance of the bit into the intact rock. 
As discussed in the previous section, for 
a sharp bit the energy consumed in ad- 
vancing through the crushed rock ahead of 
the bit constitutes the major portion of 
the energy spent. Also, from observa- 
tions of the groove left by the bit, it 
is clear that some debris is forced 
underneath the bit, where it produces 
high normal forces and, in the case of 
bits of negative rake angle which act 
mainly as indenters ( 13 ) , acts as a 
cushion that reduces the stress concen- 
trations in the rock adjacent to the bit 
and thus necessitates higher indentation 
forces. Hence removal of this debris by 
water jets should improve the efficiency 
of the cutting process. 

If erosion of crushed rock from ahead 
of the bit is the main mechanism of 
water-jet assistance, the improvement in 
cutting efficiency due to water jets 
should be controlled by the same param- 
eters that control how much debris can be 
eroded from in front of the bit. It can 
be argued that the rate of debris erosion 
is a function of the jet power dW/dt that 
can be delivered to the crushed zone 
ahead of the bit. Therefore at a given 
point along the cut, the amount of debris 
eroded should be a function of the water- 
jet energy delivered to that point, i.e., 
the amount of debris eroded per length of 
cut should depend upon the water-jet 
energy delivered per length of cut, 

dW dW 1 . pQ 

dx dt v v 

where Q is the jet flow rate. Thus if 
the main mechanism of water-jet assist- 
ance is indeed the erosion of crushed 
rock from in front of the bit, then the 



improvement in cutting efficiency due to 
water jets should depend upon dW/dx. 

Evidence of a dependence on dW/dx was 
found by Hood (13) , who measured the 
force reductions achieved with water-jet 
assistance for chisel-type bits cutting 
15 mm deep in Indiana limestone, for the 
matrix of parameter levels: 

Jet pressure p = 10, 20, 35, 50, and 

7 MPa, 

Jet diameter a = 0.65, 0.83, and 1.05 
mm, 

Cutting speed v = 0.16 and 0.42 m/s. 

Figure 5A shows the percentage mean 
cutting force reduction R c plotted versus 
dW/dx, where 



R c = = x 100 pet, 
Fco 



with F c being the mean cutting force and 
F co being the mean cutting force for a 
cut made without water-jet assistance 
(3.4 kN for these tests). The substan- 
tial overlap of the 90-pct-conf idence 
bands for the data indicates that R c is a 
function only of dW/dx when p and v are 
varied independently for the case a 
= 0.65 mm. Comparison with data for the 
other levels of a indicates that R c is a 
function only of dW/dx when all three 
parameters are varied independently. A 
similar analysis of the percentage mean 
normal force reduction R n is also a 
function of dW/dx, as evidenced by figure 
55. 

Further evidence that jet assistance 
depends upon dW/dx is provided by data 
for 10-mm-deep cuts taken at a cutting 
speed v = 1.10 m/s in Grindleford sand- 
stone, which are replotted in figure 6 
from the paper of Fowell (16). In that 
paper the confidence limits for the data 
are not given, but if variances similar 
to those seen by other researchers (14) 
are assumed, then at least up to the 
level dW/dx = 10 J/mm there is evidence 
that R c and R n depend upon dW/dx when p 
and a are varied independently. 

Further evidence in favor of the 
erosion hypothesis is provided by 



observations of the amount of finely 
crushed material remaining in the groove 
left after passage of the bit. For 15- 
mm-deep cuts with chisel-type bits in 



o 



I- 

o 

Q 
Ld 
Cd 

Ld 

o 
cc 
o 



100 
80 
60 
40 
20 




1 1 — i — i — i — i i 1 1 

A, Cutting force reduction 

KEY 

V77A 16 cm/s 
42 cm/s 




_i i i i i '''i 




10 
dW/dx, J/mm 



100 



FIGURE 5.— Force reductions achieved in Indiana limestone 
for 15-mm deep cuts. Shaded regions are the 90-pct- 
confidence bands for the data. 



I00 



u 

Q. 



o 

r- 
U 
Z) 
Q 
L±J 

or 

LU 

o 
q: 
o 



80 1- 

60 
40 
20 



1 — i — i — i — i 1 1 1 1 1 — 

Cutting force reduction 

KEY 

- a 0.6 mm 
• 0.9 mm 
■ 1. 2 mm 
o |. 5 mm 




dW/dx, J/mm 

FIGURE 6.— Force reductions achieved in Grindleford sand- 
stone for 10-mm-deep cuts. 



Indiana limestone, the authors observed 
that the amount of this finely crushed 
material diminished with increasing pres- 
sure and flow rate and with decreasing 
cutting speed. For dW/dx < 2 J/mm, the 
amount of crushed material remaining in 
the groove was not visibly less than that 
seen for dry cuts, but as dW/dx = 30 J/mm 
or so, there was almost no crushed ma- 
terial remaining in the groove. Begin- 
ning at roughly 20 J/mm, discontinuous 
pitting of the bottom of the groove was 
observed which progressed to continuous 
slotting for dW/dx > 60 J/mm or so. This 
is interesting because, as figure 5A 



shows, 20 to 30 J/mm is the range over 
which the greatest cutting force reduc- 
tions were obtained. Thus the greatest 
reductions in cutting force occur when 
most of the crushed material is flushed 
away before it can be trapped under the 
bit, but before the energy density is 
sufficient to continuously slot the rock. 
This dependence upon dW/dx might ex- 
plain why marked improvements in cutting 
rate have been observed with a roadheader 
(6_), for which dW/dx was roughly 3 J/mm, 
while no significant increase in cutting 
efficiency was observed for a shearer for 
which dW/dx was about 1 J/mm (7). 



CONCLUSIONS 



A review has been given of file pro- 
posed mechanisms of water-jet assistance. 
Of these, lubrication and suppression of 
secondary chipping are seen to contra- 
dict previously published evidence, while 
stress corrosion cracking cannot play a 
significant role at the crack propagation 
speeds prevailing in cutting operations. 
A fourth mechanism, hydraulic wedging, 
may occur, but its role is probably minor 
relative to that of a fifth, the erosion 
of crushed rock from in front of the 
bit. 

The assumption that erosion of crushed 
rock is the principal mechanism leads to 
a prediction that the force reductions R c 



and R n should depend only upon dW/dx, 
when jet pressure, nozzle orifice diam- 
eter, and cutting speed are varied in- 
dependently. This prediction is con- 
firmed by the available data. 

The observed dependence of R c and R n on 
dW/dx implies that, at least within the 
typical range of cutting speeds, there is 
no limiting speed above which water-jet 
assistance is fundamentally impossible. 
No substantial evidence has emerged to 
indicate otherwise. The hypothesis that 
the confining force exerted by the water 
jet significantly hampers cutting at high 
speeds is not consistent with the avail- 
able evidence. 



REFERENCES 



1. Hood, M. Cutting Strong Rock With 
a Drag Bit Assisted by High Pressure 
Water Jets. J. S. Afr. Inst. Min. and 
Metall. , v. 77, No. 1, 1977, pp. 79-90. 

2. Ropchan, D. , F. D. Wang, and 
J. Wolgamott. Application of Water Jet 
Assisted Drag Bit and Pick Cutter for the 
Cutting of Coal Measure Rocks (U.S. DOE 
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3. Dubugnon, 0. An Experimental Study 
of Water-Jet-Assisted Drag Bit Cutting of 
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Water Jet Symposium (Golden, CO, June 
3-5, 1981). CO Sch. Mines, Golden, CO, 
1981, pp. II.4.1-II.4.11. 

4. Tomlin, M. G. Field Trials With a 
10,000 psi Prototype System. Paper in 



Proceedings of Seminar on Water Jet As- 
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Dep. Energy, 1982, pp. Cl-Cll. 

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Experience With Boom-Type Roadheaders 
Equipped With High-Pressure Water-Jet 
Systems for Roadway Drivage in British 
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6. Barham, D. K. , and M. G. Tomlin. 
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1983, pp. 743-749. 

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1984, pp. 97-107. 

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pp. 77-106. 



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